1. Field of the Invention
The present invention relates to X-ray phase gratings and methods for producing X-ray phase gratings, and particularly to an X-ray phase grating used for X-ray phase-contrast imaging based on Talbot interference and a method for producing the X-ray phase grating.
2. Description of the Related Art
X-rays are used for transmission imaging apparatuses in the fields of industry and medicine because they have high material permeability to allow examination of the internal structure of an object. X-ray phase-contrast imaging, which is promising for reducing the exposure of a subject, has recently been researched. Among others, an imaging method using Talbot interference has been theoretically possible, which uses a transmission diffraction grating to image a phase shift from interference fringes appearing under certain interference conditions. The outline of Talbot interferometry will now be described. An example of a Talbot interferometer is shown in FIG. 10A. In FIG. 10A, an X-ray source XG, a source grating SG, X-rays XR, an object O, a phase grating PG, an absorption grating AG, and a detector D are shown. Imaging by Talbot interferometry requires at least a spatially coherent X-ray source XG, a phase-modulation diffraction grating (hereinafter referred to as “phase grating”) PG for periodically modulating the phase of X-rays, and a detector D. If the X-ray source XG has no coherence, the source grating SG is used to ensure spatial coherence.
Talbot interferometry using a source grating is also called Talbot-Lau interferometry. To ensure sufficient spatial coherence, it is necessary to satisfy the condition that the spatial coherence distance λ×(L/s) is sufficiently larger than the pitch p of the phase grating PG, where λ is the wavelength of the X-rays XR, L is the distance between the X-ray source grating SG and the phase grating PG, and s is the size of the apertures of the X-ray source grating SG. The term “pitch” as used herein refers to the period at which grating lines or channels are arranged. In Talbot interferometry, interference fringes reflecting the shape of the phase grating PG appear at a particular distance from the phase grating PG. This is called a self-image, which appears at a distance of (p×p/(2λ))×n or (p×p/(8λ))×n from the phase grating PG, where n is an integer. This distance is called the Talbot distance d. If the object O is disposed in front of the phase grating PG, a self-image of the phase grating PG formed by the X-rays XR passing through the object O contains information about the differential phase shift of the X-rays XR due to the object O. If the absorption grating AG and the detector D are disposed at the Talbot distance d to detect the self-image, a phase image of the object O can be obtained.
A cross section of a typical phase grating PG is shown in FIG. 10B. In FIG. 10B, ridges R, slits S, the height h1 of the ridges R for phase modulation, regions r0 that cause no phase modulation, and regions r1 that cause phase modulation are shown. Typically, the amount of phase modulation is constant in the phase modulation regions r1 of the phase grating PG, the ridges R have a rectangular shape, and the cross section of the phase grating PG has a rectangular structure. For example, if 30 keV X-rays are used, the height h1 of the ridges R sufficient to cause a phase shift of π is 38.5 μm for silicon. The pitch p, on the other hand, is often 10 μm or less in view of ensuring spatial coherence. Thus, the phase grating PG requires a small pitch and a high aspect ratio. In addition, the phase grating PG requires a large area. Furthermore, if the phase grating PG has a large area, the angle of the slits S with respect to the substrate needs to vary between the center and periphery of the phase grating PG. In the related art, for example, U.S. Patent Application No. 2007/0183579 and U.S. Pat. No. 7,639,786 propose methods for producing a rectangular structure having a high aspect ratio. According to these methods, sub-gratings having a low aspect ratio, which are easier to produce, are stacked to form an X-ray optical transmission grating having a high apparent aspect ratio.